Magic Numbers, Panama Edition.

After seeing Thorsten's post I was quite jealous about the flight times he was getting, so I was really excited when we got in a 53 minute flight yesterday with a new modified copter design here in Panama.

I am down in Panama on a post-doc with the Smithsonian Tropical Research Institute (STRI) to use copters, Ecosynth, and other remote sensing and computer vision technologies to improve our understanding of tropical forest canopies. My stay here is supported by a STRI grant that emerged directly from my work here last year.

I came here geared up to build two Ecosynth-style (aka Gienow-style) Mikrokopter / Arducopter -hybrid copters in an Octocopter configuration. My build consists of MK frames and 3DR / Pixhawk electronics along with MaxAmps 22000 mAh 14.8V lipos, with which we were getting solid 34 minute missions with about 9 minutes of battery life to spare before the alarm started buzzing at 3.3V per cell. This was enough time for us to carry out a round trip 12.5 km mission while at the same time traveling up to 330m altitude and back (ceiling here is ~600 m) carrying a standard Ecosynth payload of Canon ELPH 520HS, Canon WP-DC340L Waterproof case (no longer in stock in the Universe :( ), and Garmin Astro DC50 dog tracker - total weight about 600g. Assembled copter pics here and here.

And we were getting in some great missions over 'The Island' up until I moved the DC50 GPS antenna to try to improve signal which I think resulted in a modified electrical field around the 3DR compass, resulting in a wicked crash, so sad. Fortunately it was the improved signal of the DC50 that allowed us to recover the copter over 1km away in jungle. 1500 hectares of forest and trees and it lands in a creek! We lost 7 of 8 arms, all the props, both center plates, a battery buzzer, and a landing leg, however I plunged all the electronics into tubs of dry rice for 3 days (camera included) and it appears that all of that is working ok.

This leads us to the new long flight times. I lost 7 arms and only had 6 new spares (it was all the stock in the store at the time of my purchases). Struggling to find a solution for the rebuild, I thought, "Why not a Quad?" These motors should produce enough lift for our standard payloads and taking off four arms (motors, escs, arms and props) saved 800 grams total. The existing Gryphon PDB made the new config easy. After a total reconfig of the parameters (Pixhawk firmware, accelerometer, compass, compassmot, PID Autotune, which by the way are all getting super easy, thanks Ardu community!) we were up and flying. We strapped on a 22000 mAh lip and carried out our 12.5 km test mission over our test area (18 700m length laps).

The new quad (Mediaocho "half of eight") performed well, especially considering the high winds (about 12 km/hr). It completed the main mission in about 34 minutes, just like the Octo (same WP_NAV speed, 7 m/s) and then just before landing in RTL mode I switched to Loiter and just waited. And waited. 43 minutes passed (our max flight time with the Octo on the same battery and same mission) and it was still flying. It kept going until the buzzer started going off at about 52:50, and by the time I brought it down to a stop and zero throttle, the timer was at 53 minutes.

We are thinking that this will allow us at least 15km missions, including up and down 300m of elevation. It might even have slightly more flight time considering Translational Lift, which Stephen tells me could make it more efficient for the copter to fly long straight lines than to hover in one place. In addition, the copter makes 74 turns in our test mission, each one costing about 5 seconds were the copter has to slow down to about 2 m/s to get through the turn, whcih is about 5 times more turns than we make in a normal mission and which might be costing us another 5 minutes! Does this mean we have a potential maximum of one hour flight time? I can't wait to find out.

Jonathan, two more questions: what was the final voltage (in air) and how many mAh were left after the 53 min? I found that the remaining mAh seems to be the better and more secure* measure compared to the voltage. Normally, I let it run down to 20% of the capacity. This seems to be consistent between different batteries. However, the remaining voltage at that percentage varies between 13.8V and 14.3V with my batteries. So, if you drain the 14.3V (at 20%) battery further down 13.8 V you might destroy it. So it would be interesting how the MaxAmps behaves?

* for sure only if you use a fully charged battery! If your batteries are not fully charged the voltage is important - but needs to be adjusted with respect to the specific battery used.

I agree that the voltage is low. The battery alarm starts beeping when anyone cell hits < 3.3V. So it was probably more like 13.5V or so total. I agree that this would be a maximum flight time. I would not plan to fly an entire mission of this length, but perhaps one at 43 minutes with a good buffer. I plan to test a 15.6 km mission this week which should take about that long, will let you know how it goes.

Also, I have also started switching the lipo-charger to balance mode after it says it is at max voltage in charger mode. This can sometimes take another two hours at low amps (0.1A), but I also think this gives us a much more fully charged battery. The Maxamps definitly have a good weight to amps ratio, their 11000 and 22000 mAhs 14.8V lipos are at 12.8 mAh /gram. I'd like to try some of those Panasonic li-ions that folks are using for long endurance missions.

Yes, the Panasonics are interesting. But it makes building the system more complex. Not sure about the costs and maintenance. The MaxAmps are very expensive as well. However, they are very interesting. The hexa should fly about 65 min. I need to run some tests with higher payloads.

I recently reproduced Jonathan's build with only slight modifications:

Quad with same motors (MK3638), props (APC 14" x 4.7 Slow Flyer) and ESCs (JDrones 30A) and hilander legs. I added Mikrokipters angled motor mounting plates that are supposed to improve yaw but no idea how important these are.

I used slightly shorter Mikrokopter riggers (350mm). RX is FrSky X8R. GPS is Neo 7. I also have 3DR Telemetry on board.

I slung the Pixhawk (Fixhawk to be precise - RCTimer version) on a dampened platform under the centre plates which I also plan to attach a camera to. The Mikrokopter Octo XL frame kit includes lots of useful spare nuts, bolts and spacers so that I could easily build extra platforms and other adaptations to suit my particular set up.

I used a cheap octo pdb with bullet connectors so I had the option to add motors if desired and more easily dissemble for transport.

This setup without battery or camera weighs 1300g. It's hard to see how to make it much lighter. The shorter Mikrkopter riggers (290mm) would save 40g and the hilander legs could be cut a bit shorter but these would add little to endurance.

A hover endurance test with a single 4S 6600 mah Multistar (all up 1855g) flew for 30 minutes before landing with the battery on 13.8v under load/14.6v recovered (which equated to 80% used). This was pretty much exactly as predicted from the motor curves on the mikropkopter website here. Strangely though those curves suggest only about 37 minutes for Jonathan's spec using the 22K maxamps battery unless I'm missing something. Not quite sure why that would be so far off. I also found ecalc to produce a poor prediction for my setup. It suggested hover amps for the four motors of 15.5A with a 6600 battery whereas reality was about 10A. Perhaps I am getting something wrong with these calcs.

The batteries seem good - thanks for the advice Jonathan. Very light for their capacity. 6600mah = 547g. I make that 12.1 mAh/g. They are only 10C but not sure that matters. The only difficulty is getting hold of them.

The biggest surprise for me was how quiet the slower motors/props were compared with the whiny/buzzy F550 I have been practicing on to date.

I've also been flying it without any tuning at all as I couldn't get autotune to work (yet). The only significant change I've had to make to the parms so far was to increase the minimum throttle to 150 because at 130 the motors would stop spinning predictably when dropping the throttle quickly.